1. Field of the Invention
The present invention relates to plasma processors, and more particularly to a plasma processor which is well suited to process a sample, such as a semiconductor substrate, with a plasma under a cooled state.
1. Description of the Prior Art
As techniques for processing a sample, such as a semiconductor substrate, with a plasma under a cooled state, there have heretofore been known ones as disclosed in, for example, the official gazettes of Japanese Patent Applications Laid-open No. 72877/1988, No. 110726/1988 and No. 174322/1988.
Disclosed in the official gazette of Japanese Patent Application Laid-open No. 72877/1988 is the technique wherein a sample holder which is cooled with cooling water is disposed in a vacuum vessel, an electrode for an electrostatic chuck coated with a dielectric film is mounted on the sample holder, the object to-be-processed is electrostatically fixed to the electrode for the electrostatic chuck by applying a DC voltage to this electrode, and the rear surface of the fixed object to-be-processed is fed with a cooling gas from a cooling gas conduit which is connected penetrating the electrode for the electrostatic chuck, whereby the efficiency of cooling the object to-be-processed is heightened for the plasma process. In this case, the plasma is generated in such a way that an RF (radio-frequency) voltage is applied between the sample holder and an electrode opposing thereto by an RF power source which is connected to the sample holder.
Each of the official gazettes of Japanese Patent Applications Laid-open No. 110726/1988 and No. 174322/1988 discloses the technique wherein a sample holder which is cooled by a coolant is arranged in a processing chamber, and a heating element is provided in the sample holder, whereby the temperature of the sample placed on the sample holder is controlled for the plasma process by the heating element and the coolant. In this case, the plasma is generated in such a way that an RF voltage is applied between the sample holder and an electrode opposing thereto by an RF power source which is connected to the sample holder. Alternatively, it is generated with a microwave. Regarding the method which generates the plasma with the microwave, the sample is sometimes subjected to the plasma process by connecting an RF power source to the sample holder and inducing a bias voltage in this sample holder.
The prior-art techniques mentioned above (in the official gazettes of Japanese Patent Applications Laid-open No. 72877/1988, No. 110726/1988 and No. 174322/1988) have not taken it into consideration that the coolant to be used and the materials of a pipe for feeding the coolant, are altered depending upon the temperature regions of cooling. More specifically, in these prior-art techniques, it is common practice that a bias voltage based on RF power or the like is applied to the sample holder in the plasma process, herein, etching process of the sample. However, in such a case where the sort of the coolant has changed or where the cooling temperature is low, a metal pipe being electrically conductive is sometimes used as the pipe for feeding the coolant, and the RF power might leak from the metal pipe. Moreover, in the case of the change of the sort of the coolant the RF power might leak through the coolant because electrical characteristics differ depending upon coolants.
The former prior-art technique (in the official gazette of Japanese Patent Application Laid-open No. 72877/1988) has not considered the efficiency of the conduction of heat in the case where the heat of cooling from the sample holder cooled by the cooling water is transmitted to the sample to-be-processed fixed to the electrode for the electrostatic chuck, through the electrostatic chuck electrode coated with the dielectric film. More specifically, the technique has the problem that the interposition of the electrostatic chuck electrode between the sample holder and the sample to-be-processed gives rise to microscopic gaps between the sample holder and the electrostatic chuck electrode, so the heat of cooling is not effectively transmitted from the sample holder to the electrostatic chuck electrode. This is ascribable to the fact that, as the cooling temperature lowers, heat transfer cannot be expected of radiation heat and cannot help being resorted to conduction heat. The problem is serious especially in an apparatus wherein the sample is processed by cooling the sample holder to a temperature below the freezing temperature of water (below 0.degree. C.). Further, a microscopic gap exists in the contact surface between the sample holder and the electrostatic chuck electrode, and it does not become substantially uniform, so that the thermal conductivity of the whole surface varies. In addition, the states of the contact surfaces differ due to discrepancies involved in the apparatuses, so that the individual apparatuses exhibit unequal thermal conductivities. This leads to the problem that the temperatures of the samples during the plasma processes become non-uniform to incur a dispersion in the qualities of the processed samples, for example, wafers among the processes, lots or processors of these wafers. In the case where the samples are cooled to the temperature below the freezing temperature of water and then processed with the plasma, the chemical reactions between the samples and radical species in the plasma are prone to depend upon temperatures. Therefore, when the temperatures of the samples become non-uniform as stated above, the non-uniform processes appear drastically.